1. Field of the Invention
The present invention generally relates to toner concentration control technique for use in electrophotographic equipment.
2. Description of the Related Art
Detection of toner concentration using the electric conductivity of liquid developer has been proposed in in Japanese Patent Unexamined Publication No. 3-295453. The electric conductivity is measured using alternating current because measurement using direct current causes movement of ion carriers and polarization which causes voltage drops around the electrodes. The measurement frequency is determined depending on the frequency response of the object. In the case of liquid developer, a frequency of 1 kHz may be preferably used.
However, the number of ionic contaminants or the like increases due to deterioration of liquid developer. Such ionic contaminants become a factor that substantially influences the measurement, resulting in a lower degree of measurement accuracy.